1. Field of the Invention
The present invention relates to an ethylene polymer based composition and more particularly to a low pressure ethylene polymer based composition having a decreased tendency to undergo an increase in the viscosity thereof during the hot, or melt, compounding thereof.
2. Description of the Prior Art
During the hot compounding of polyolefin resin based compositions, antioxidants are often added to inhibit the thermal oxidative degradation of the resin, which can cause a decrease in the viscosity of the composition.
During the hot compounding of low pressure polyethylene based compositions an increase in the viscosity of the composition sometimes occurs due to crosslinking between polymer molecules which leads to an increase of the average molecular weight of the polymer, and/or an increase of the high molecular weight tail fraction of the polymer. This increase in viscosity may lead to the need for additional energy requirements during the subsequent conversion of the composition into commercial articles, as for example, pipe, bottles, film, tape, profiles and blow molded and injected molded articles. The deleterious effects of such a viscosity increase, such as increased requirements for energy, mechanical degradation of the resin due to the use of higher shear forces, and a decrease in throughput through the compounding equipment could be limited to some extent by increasing the compounding temperature being employed. Such a temperature increase, however, is not entirely acceptable because it tends to lead to, for example, a tendency of the polymer to discolor, and the creation of bad odors. Further, additives that are commonly used in the polymer have a critical decomposition temperature which can not be exceeded and this also tends to limit an increase in the compounding temperature. In addition, there are limitations in the heating capacity of various types of the hot compounding equipment which can be employed.
The viscosity increase which can occur during the hot compounding of polyethylene resin based compositions tends to be much more severe when the resin employed is a high molecular weight low pressure polyethylene. The development of the use of low pressure polyethylene for new applications during recent years has focused on the use of the higher molecular weight polymers. Thus, since a viscosity increase during the hot compounding of the resin tends to become more severe for low pressure polyethylene resins having the higher molecular weights, the practical difficulties involved in hot compounding such resins have increased more and more.
For some high molecular weight resin based compounds it is thus impossible to readily incorporate additives therein by conventional hot compounding techniques. Instead, the method chosen to incorporate the additives is to dry blend them with the polymer at a temperature below the melting temperature of the polymer. This, however, among other things, tends to provide a nonhomogeneous distribution of the additive in the blended composition. Further, if such a dry blended low pressure polyethylene based composition is then converted to, for example, containers in blow molding equipment, the molecular weight of the resin in the finished container is much higher than that of the original polymer due to crosslinking of the polymer during the blow molding process.
Tests with conventional antioxidants and other known additives show that these materials provide little or no help in preventing an increase in viscosity of these compositions under these molding conditions. This can be interpreted to mean that the conventional antioxidants cannot stop the usual crosslinking reactions which are believed to be induced by the presence of free radicals.